2.8 UV editing, can't box select polygons in 3D viewport anymore
The answer is to delete the UV Editing workspace and make a new one.

Big thanks to Jachym Michal for the solution in a comment.

Steps


On the tabs across the top of the window (these are Workspaces), right click on UV Editing
Select delete





Click the [+] icon on the right of the workspace tabs
Navigate to General ➡ UV Editing




You'll have a new UV Editing Workspace. You might need to zoom in on your UV and re-apply any 3D viewport settings you had.